Thermionic electrode for discharge lamps



April 10, 1962 J. E. WHITE THERMIONIC ELECTRODE FOR DISCHARGE LAMPSFiled March 29, 1960 I 5 4M 4 m e III! is: E N E $5 z V I. E I iiiwia L0 M I V% I 5 F [m m L 6 HA m 5 n I F. 2 5 M E P J M M Y w A B M 2 0 6 2n w w w w w o w m B n u U0 WWSkwQWkSQk wQQwR uhww ll/J ATTORNEY UnitedStates PatentCl 3,029,359 THERNHONIC ELECTRODE FOR DISCHARGE LAMPS JohnE. White, Cleveland Heights, Ohio, assignor to General Electric Company,a corporation of New York Filed Mar. 29, 1%0, Ser. No. 18,302 7 Claims.((Zi. 313-185) :This invention relates to high current electricdischarge lamps and more particularly to a thermionic electrode for usein same.

There has recently been developed an intermediate pressure gas dischargelamp which operates with high pressure characteristics and wherein thedischarge is wall stabilized. This lamp is described and claimed inPatent 2,924,733, Schirmer et al., issued February 9, 1960. The fillinggas in this lamp ordinarily is xenon and the discharge therethroughfills substantially the entire cross section of the envelope. The gastemperature is only a little less than the electron temperature, asevidenced by a strong continuum in the spectrum. These results areachieved by operating the lamp at a very high current. For instance, ina lamp having a tubular envelope with an internal diameter ofapproximately 3 centimeters and designated XBL 16,000, the dischargecurrent is approximately 75 amperes R. M.S. The thermionic electrodes ofthe present invention are designed to support discharge currents of thisorder of magnitude and are intended primarily for use in this type oflamp.

The thermionic electrodes or cathodes originally used in wall stablizedhigh current lamps consist of hollow bullet-shaped bodies of thoriatedtungsten. The rounded front end or nose of the tungsten body wasprovided with several narrow slits and within the body a pellet ofthorium oxide was inserted. In operation, some thorium oxide is reducedby the tungsten of the electrode body and thorium metal diifuses throughthe narrow slits to the outer surface of the tungsten body. The workfunction of the tungsten body is thereby reduced to facilitatethermionic emission. However it has been observed that with suchelectrodes the lamps darken quite rapidly. -At the rated load of 75amperes R.M.S. the electrodes run very hot, for instance atapproximately 2250 C. corresponding to a white heat. Calculationsestablish that at this temperature, the electrodes radiate approximately700 watts each. Such a large electrode loss can hardly be explained'onthe basis of power delivered during the anode half-cycle, and mustresult from the presence of a substantial pro portion of ion currentduring the cathode half-cycle. It thus appears that the current densityavailable from the thorium-thoria film near the electrode slits underthermionic conditions of emission is insufiicient to meet the circuitrequirements. Therefore the proportion of current carried by positiveions goes up and the electrode is heated up by ion bombardment. ionbombardment sputtering and thermal evaporation therefore appear to bethe major factors which produce early bulb blackening in these lamps.

The object of the invention is to provide a new and improvedself-heating thermionic cathode particularly suitable for operation athigh currents in a gas or vapor discharge medium.

A more specific object of the invention is to provide a thermioniccathode capable of supplying the current requirements of a wallstaolized gas discharge lamp operating with high pressurecharacteristics, and achieving substantially improved maintenance andreduced bulb blackening.

Still another object of the invention is to provide a thermionic cathodehaving the above-described characteristics and which supplies thecurrent requirements on the cathode half-cycle with a low ion componentand at a substantially lower temperature in order to achieve higherefficiency and a relatively long-life lamp.

In accordance with the invention, a thermionic selfheating cathodeachieving the above objects is composed of a hollow cup-like body of arefractory metal such as tungsten or molybdenum, open towards the front,that. is in the direction of the arc; a coil of a refractory metal wire,again tungsten or molybdenum, is located within the body cavity incontact with the wall; emissive material consisting of an alkaline earthmetal-containing compound is placed on the coil within the cavity andlodged in the interstices between the turns of the coil and the wall ofthe body. In a preferred embodiment, the body and coil both consist oftungsten and the emissive material consists of barium thorate. Strontiumthorate is also suitable.

Preferably according to the invention, the ratio of depth to diameter ofthe electrode cavity is at least 1:1 or more. Surprisingly, despite thedepth of the cavity, the arc does not hot spot on the first few turnsbut remains diffuse and extends all the way into the cavity to the lastturns of the tungsten coil therein, The cathode operates at a much lowertemperature and with a very low cathode drop and correspondingly lowlosses. For instance a fivefold reduction in cathode loss over. thatobtaining with the prior type of electrode has been achieved.

For further objects and advantages and for a. better understanding ofthe invention, attention is now directed to the following description ofa preferred embodiment of the invention and to the accompanying drawingillustrating same. The features of the invention believed to be novelwill be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 illustrates a wall stabilized high current gas discharge lampincorporating thermionic electrodes in accordance with theinvention.

FIGS. 2 and 3 illustrate a cathode construction em- [bodying theinvention.

ous conditions of operation.

Referring to the drawing and more particularly to FIG. 1, theillustratedlamp 1 is intended for AC. operation. It comprises a tubularenvelope 2 made of quartz, shown herein partly sectioned, and containingXenon as the filling gas. At the ends of. the envelope are mounted thethermionic electrodes 3 supported on rod-like conductors 4 which extendthrough reduced tubular quartz extensions 5. Beyond the tubularextensions 5, graded seals 6 consisting of vitreous sections withintermediate coefiicients of expansion terminate in glass sections 7 towhich are sealed thin-walled metal thimbles 8. The thimbles 8 are madeof a suitable metal for sealing to the glass and the .outer ends of theconductors '4 extend through and are welded or brazed to contact buttonsor terminals 9 on the ends of the thimbles.

The construction of the electrodes 3 is best seen in FIGS. 2 and 3illustrating a preferred embodiment. The electrode proper comprises agenerally cylindrical cupshaped body 10 of tungsten open towards thefront, that or barium zirconate. I have found barium thorate accordingto the formula BaThO satisfactory and apply the material as a suspensionin nitrocellulose. The material may be applied with a brush, then driedand fired to sinter it.

Preferably the forward end of the tungsten coil is turned axiallyoutward as indicated at 12 in FIGS. 2 and 3 in order to projectforwardly of the electrode body in the direction of the arc. Theforwardly projecting tip 12 may be located immediately at thecylindrical cavity wall of the electrode body as best seen in FIG. 3.Alternatively, the end of the coil may be turned radially in towards thecenter line of the electrode body, as indicated at 13 in FIGS. 4 and 5,and the end of the extension turned forwardly at 14 so as to projectalong the axis of the electrode. The arrangement of FIGS. 2 and 3 is thesimpler one and facilitates the application of the activation materialto the coil while the arrangement of FIGS. 4 and results in greatersymmetry.

In actual tests of electrodes constructed as illustrated, certain highlyadvantageous features have been established. Firstly the use of aforwardly projecting coil tip projecting slightly beyond the electrodebody permits ignition of the are at a preferred location, that isdirectly on the electrode coil. Ignition at this point is preferredbecause when the arc is initially cold started, the emission is fieldemission rather than thermionic emission. It follows that the activatingmaterial at the ignition point will, at least in part, be sputtered off.However the electrode wire operates at a higher temperature than theelectrode body so that, especially at the tip, there is but a trace ofemissive material on it, of the order of a monolayer. As a result, theamount of emissive material sputtered off at starting is minimized.Since the electrode coil is thermally shielded by the electrode body, itcan be heated up with much less energy than would be required to heat upthe entire electrode body and thus a lower power starting circuit can beused. I have observed that at starting the electrode coil becomesincandescent much before the electrode body and therefore goes into thedesirable thermionic mode of emission long before the body.

A feature of the present electrode is that emissive material located inthe electrode cavity diffuses over its inner surface and over thesurface of the electrode coil making the entire concave regionthermionically emitting at a relatively moderate temperature. It iswell-known that at temperatures sufficient for surface migration ofemissive material such as barium metal, evaporation is imminent. In anelectrode wherein the exterior surface is the electron emitting surface,vaporization of emissive material is relatively rapid.

For instance, in the bullet-shaped type of electrode originally usedwith wall stabilized high current gas discharge lamps, the emissivematerial diffusing out through the narrow slits in the electrode wouldmigrate on the average only a short diffusion length before becomingevaporated and totally lost to the lamp wall. However with the presenthollow electrode, much of the evaporation proceeds only to the otherside of the cavity and the evaporated material is there recaptured andremains available for further aid in emission.

Certain electrical effects supplement the geometry of the instantconcave electrodes in conserving the emissive material. In thehalf-cycle during which the electrode is operating as anode,electrophoretic transport by electrons reduces the migration of neutralatoms away from the anode. When the electrode is operating as cathode,such of the atoms as are ionized are returned to the cavity under theaction of the electric field. This serves to reduce further the loss ofemissive material and blackening of the envelope wall.

One of the surprising features of the present electrode is the fact thatit operates in a diffused mode without the formation of any hot spot atany point. The are extends even to the innermost turns of the electrodecoil within the cavity and this fact has been verified experimentallyusing electrode probes. These probes were inserted right into theelectrode cavity through small holes pierced through the electrode wallfor that purpose. The cathode potential drop has been measured and hasthe extremely low value of 1.2 volts peak at amperes R.M.S. This issurprisingly low, especially when compared with the 7.5 volt cathodedrop of prior art cathodes as used in Xenon discharges reported in theliterature.

It is advantageous to make the depth D of the cathode cavity as great aspossible provided the are extends into it in a diffuse mode. Whether theare will so extend is governed in part by the ratio of cavity depth 10diameter D/d, where d is the diameter of the cavity measured within theelectrode coil 11. In one cathode which I have constructed and tested,the ratio D/d was approximately 1:1. This cathode supported a current of75 amperes A.C. (R.M.S.) and operated at a temperature of approximately1600 C., the loss per electrode being about 250 watts. In anothercathode wherein the ratio D/d was approximately 2.7:1, a current of 75amperes A.C. (R.M.S.) was supported with a cathode drop of 1.2 volts.The cathode operated at a temperature of 1430 C., corresponding to aloss of approximately watts per electrode. In this latter cathode havingthe physical configuration illustrated in the drawing, cavity depth Dwas 0.713, outside diameter of electrode body was 0.491, and overallcavity diameter was 0.366". The electrode coil consisted of .050"diameter tungsten Wire, the diameter d of the cavity defined by theelectrode coil was 0.266". Thus the ratio D/d was approximately 2.7:1.

In general, a ratio of cavity depth to diameter D/d in the range of 1:1to 4:1 is desirable in accordance with the invention for thermionicemission in the diffuse mode with minimum electrode losses and leastvaporization or sputtering of emissive material. For use in a wallstabilized gas discharge lamp operating with high pressurecharacteristics, a ratio D/ d in the range of 2:1 to 3:1 is preferred.The illustrated electrodes, wherein the ratio D/d is 2.7:1, were testedin a wall stabilized discharge lamp having a filling of xenon at apressure of about millimeters of mercury and having an operatingpressure of about 1.4 atmospheres at a current intensity of 75 amperesA.C. (R.M.S.). A threefold reduction in the rate of envelope blackeningwas observed, and the electrode loss of 140 watts per electroderepresented a fivefold reduction from that obtaining with priorelectrodes.

A feature of the present electrode is that substantially no heatingoccurs on the cathode half-cycle of conduction. This is due to the factthat the cooling effect due to electron emission offsets the heatingeffect due to concurrent ion current. These results are shown by thecurves of FIG. 3 wherein curve 20 shows the electrode temperatureagainst current for normal A.C. operation wherein the electrode operatesas cathode on one half cycle and as anode on the other half cycle. Curve21 shows the temperature-current relationship for half-wave anodeoperation wherein the current is rectified by external circuit means andthe electrode conducts on the anode half cycle only. It will be observedthat the electrode temperatures in both cases are very close. Thereforeon normal A.C. operation, the cooling effect due to electron emission onthe cathode half cycle must approximately balance any heating effect dueto ion current during that half cycle. Thus electron emission is seen tooccur in a very efficient mode.

The preferred embodiment of the invention which has been describedherein is intended as exemplary and not as limitative of the invention.Various modifications will readily occur to those skilled in the art.The appended claims are therefore intended to cover any suchmodifications coming within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A high current thermionic self-heating electrode comprising a hollowcup-shaped refractory metal body open at one end, a coil of a refractorymetal Wire located within said body and lining the inside walls thereof,an electron emitting compound coated on said coil and on the interiorwalls of said body, said electrode defining a cavity having a ratio ofdepth to diameter at the open end of at least 1:1.

2. A high current thermionic self-heating electrode comprising a hollowcup-shaped refractory metal body open in the direction of the arc to besupported thereby, a coil of a refractory metal wire located within saidbody and lining the inside walls thereof, an electron emitting compoundcoated on said coil and on the interior walls of said body and fillingthe interstices between the turns of said coil and the walls of saidbody, said electrode defining a cavity having a ratio of depth todiameter at the open end of at least 1:].

3. A high current thermionic self-heating electrode comprising a hollowcup-shaped refractory metal body open in the direction of the arc to besupported thereby, a coil of a refractory metal Wire located within saidbody and lining the inside walls thereof, an electron emitting alkalineearth metal compound coated on said coil and on the interior walls ofsaid body and filling the inter stices between the turns of said coiland the Walls of said body, said electrode defining a cavity having aratio of depth to diameter at the open end in the range of 1:1 to 1:4.

4. A high current thermionic self-heating electrode comprising a hollowcup-shaped refractory metal body open in the direction of the arc to besupported thereby, a coil of a refractory metal wire located within saidbody and lining the inside walls thereof, an electron emitting alkalineearth metal compound coated on said coil and on the interior walls ofsaid body and filling the interstices between the turns of said coil andthe Walls of said body, said electrode defining a cavity having a ratioof depth to diameter at the open end in the range of 1:1 to 4:1, saidcoil having a tip projecting forwardly of said body a slight distance inthe direction of the arc.

5. A high current thermionic self-heating electrode comprising a hollowcup-shaped tungsten body open in the direction of the arc to besupported thereby, a coil of tungsten wire located within said body andlining the inside walls thereof, an electron emitting alkaline earthmetal compound coated on said coil and on the interior Walls of saidbody and filling the interstices between fractory metal body open at oneend, a coil of a refractory metal wire located within said body andlining the inside walls thereof, an electron emitting compound coated onsaid coil and on the interior walls of said body, said electrodedefining a cavity having a ratio of depth to diameter at the open end ofat least 1:1.

7. A wall stabilized high current discharge lamp operating with highpressure characteristics comprising an envelope containing a filling ofxenon and having a pair of thermionic self-heating electrodes sealedinto opposite ends, each electrode comprising a hollow cup-shapedrelatively massive tungsten body open in the direction of the arc to besupported thereby, a coil of tungsten wire located within said body andlining the inside walls thereof, an electron emitting alkaline earthmetal compound coated on said coil and on the interior walls of saidbody and filling the interstices between the turns of said coil and thewalls of said body, said electrode defining a cavity having a ratio ofdepth to diameter at the mouth in the range of 1:1 to 4:1, the coil ineach e1ectrode having a tip projecting out of the electrode cavity inthe direction of the other electrode.

References Cited in the file of this patent UNITED STATES PATENTS FranceFeb. 16, 1955

1. A HIGH CURRENT THERMIONIC SELF-HEATING ELECTRODE COMPRISING A HOLLOECUP-SHAPED REFRACTORY METAL BODY OPEN AT ONE END, A COIL OF A REFRACTORYMETAL WIRE LOCATED WITHIN SAID BODY AND LINING THE INSIDE WALLSTHEREOF,AN ELECTRON EMITTING COMPOUND COATED ON SAID COIL AND ON THEINTERIOR WALLS OF SAID BODY, SAID ELECTRODE DEFINING A CAVITY HAVING ARATIO OF DEPTH TO DIAMETER AT THE OPEN END OF AT LEAST 1:1.